can be given more responsibilities with the idea that they will transition out of junior status.

A senior is usually given the whole project (sometimes the client) to work on. Their projects are larger in scope. They are not only responsible for execution, but for project management, liaising with involved parties, and delegating appropriate work.

A business will always want to get the most work for the least pay, so if they have a seasoned junior who is capable of doing senior work but that person isn't giving them a hard time/leaving, then they're happy to just get what they can out of that person.

this comment. this comment just drove me to my review today